The present invention relates to enhancing the depth of field of an optical imaging system independent of its F# (f number).
The performance of all pattern recognition and tracking systems is limited by the depth of field of the optical imaging system used to acquire the images. Conventional optical systems require the aperture stop to be reduced in order to achieve a higher depth of field. Reducing the aperture causes the exposure (flux time integration time) to be reduced. In addition, reducing the aperture stop causes higher spatial frequencies of the object to be attenuated, thus degrading the image. Other image processing techniques have been used to overcome these obstacles. However, to date, all such image processing techniques to produce enhanced depth of field images are based on numerically intensive mathematical image processing algorithms that cannot be implemented in real time. Hence, such techniques are used as a post processing technique to produce images with an enhanced depth of field.
Adaptive optics technology does not fall neatly into any of the established engineering disciplines; it combines elements from optics (imaging systems and interferometry), electro-optics (photon sensing and modulation devices), electrical engineering, mechanical engineering, and chemistry. The use of active and adaptive optics terminology is not standard throughout the optic community. The literature often confuses and interchanges the usage of these terms. For example, as pointed out in Robert Tyson""s text entitled xe2x80x9cPrinciples of Adaptive Opticsxe2x80x9d, Academic Press, New York, 1991, many researchers differentiate them by bandwidth. They refer to systems operating below {fraction (1/10)} Hz as active and those operating above {fraction (1/10)} Hz as adaptive. This definition is used widely in the astronomy community. Other restrictions on the definition of adaptive optics have been seen. Many people skilled in the art consider adaptive optics to be restricted to coherent phase-only correction. The definition we will use to describe this invention, and it is one that is gaining more and more popularity, is that everything having to do with actively controlling a beam of light is active optics. As discussed in Tysons aforementioned book entitled xe2x80x9cPrinciples of Adaptive Opticsxe2x80x9d, adaptive optics is a subset of the much broader discipline, active optics. The terms adaptive and active optics in this invention will be used as many contemporary workers skilled in the art tend to do throughout the current literature. The term adaptive optics is used when it specifically applies to wavefront sensing (sensing aberrations) and/or wavefront correction. In this invention, the broader term active optics is used to describe devices that can be used to control lightxe2x80x94such as tunable filters, programmable waveplates and other spatial light modulators.
As discussed is several optical information-processing patents, pattern recognition is extremely sensitive to a variety of distortions including defocus. U.S. Pat. Nos. 5,485,312 and 5,111,515 are all optical information processing systems that can benefit from this invention that improves the depth of field. As discussed in this disclosure, all other optical information processing systems used for pattern recognition and tracking suffer from this drawback. This invention overcomes this obstacle by enabling a single image with a large depth-of-field to be used instead of trying to recognize or track out-of-focus objects.
As discussed in U.S. Pat. No. 4,141,652, various adaptive optics systems have been devised to improve resolution by correcting for distortions induced in light wavefronts by atmospheric disturbances and imperfections of the receiving optical systems. U.S. Pat. No. 4,141,652 relates to improvements in the Hartmann-type sensors. (See also U.S. Pat. Nos. 4,399,356 and 5,120,128.)
U.S. Pat. Nos. 4,935,614 and 5,026,977 describe double pass phase shifting interferometric adaptive optic systems which only operate with coherent light.
Other phase diversity techniques such as those disclosed in U.S. Pat. Nos. 5,384,455, 4,308,602 and 5,610,707 are based on numerically intensive algorithms and an adaptive optics post processing technique because they cannot be implemented in real-time imaging systems. Moreover, these phase retrieval techniques require sufficient spatial frequency terms in order to operate. Global convergence for extremum values is difficult to achieve. In addition, these inventions operate only on incoherent imaging systems and hence are limited in their application.
Although there are several patents awarded in the field of optical information processing, none take advantage of using adaptive optics to produce well-defined targets by enhancing the optical depth of field. Likewise, no other inventions in the field of adaptive optics is used to enhance the depth of field. Techniques reported in the literature to enhance the depth of field are post-processing techniques that cannot be implemented in real-time.
It is therefore an object of the present invention to significantly enhance imaging, pattern recognition and tracking of a very wide range of imaging systems, such as cameras, microscopes, machine vision systems, optical correlator systems, etc., especially in real-time.
The inventive optical imaging system comprises a plurality of photo-dector elements; means for measuring the amount of defocus aberration for each photo-detector element; and means for compensating for such aberration to produce an image with an enhanced depth of field independent of f number.
Whereas in conventional optical systems the depth of field depends on the focal number of the particular optical system, the inventive system enables all objects within the field of view to be viewed in focus. The novel active and adaptive optic techniques utilized by the present invention compensate for spatial and chromatic aberrations and consequently enable a large depth of field independent of the optical system""s F#, especially in real-time. The enhanced depth of field in turn greatly enhances the ability of optical information processing systems to recognize and track patterns.
This invention is a novel method of enabling an optical imaging system to have a large depth of field independent of F#. The F# of an optical system is f/D, where f is the effective focal length and D is the diameter of the exit pupil of the optical system. It is well known that all imaging systems can be characterized in terms of its stops and pupils. The aperture stop is the element in an imaging system that physically limits the angular size of the cone of light accepted by the system and it therefore governs the total radiant flux reaching the image plane. It may be simply the edge of one of the lenses in the system, or it may be an opaque screen with a hole in it specifically introduced for that purpose. In a camera, the iris diaphragm acts as an aperture stop with a variable diameter.
The field stop is the element that physically restricts the size of the image (or field of view). It may be an opaque screen with a hole in it specifically introduced for that purpose, or as in a camera the film may effectively serve as the field stop. The entrance pupil is the image of the aperture stop, as viewed from object space, formed by all of the optical elements preceding it. Frequently it is a virtual image and thus is the xe2x80x9capparentxe2x80x9d limiting element for determining the angular size of the cone of light accepted by the system. The exit pupil is the image of the aperture stop, as seen from image space, formed by all of the optical elements following it. The aberrations of a system, as well as its resolution, are often associated with the exit pupil. Ideally, for a point object, a spherical wave is launched by the exit pupil and converges to an ideal point image.
Any ray that emanates from an off axis object point and physically passes through the center of the aperture stop is called the chief ray. A chief ray is directed toward the center of the entrance pupil as it enters the system and appears to emanate from the center of the exit pupil as it leaves the system. Any ray emanating from an on axis object point that physically grazes the rim of the aperture stop is called the marginal ray. A marginal ray appears to be directed toward the edge of the entrance pupil as it enters the system and appears to emanate from the edge of the exit pupil as it leaves the system. It is noted that an image plane is located at every axial position where the marginal ray crossed the optic axis, and that the height of the chief ray at such a point determines the height and magnification of the corresponding image.
The above discussion so far has been based on the assumption that all of our imaging systems exhibit ideal behavior, i.e. they cause a point object to be mapped into a point image at the proper location according to well known geometrical optics. In practice however, we may encounter aberrations that cause no ideal images to be formed, and these aberrations may be of such magnitude that they seriously degrade the image (even after diffraction effects are accounted for). The subject of optical aberrations is quite complex.
We may divide aberrations into two general categories: those that are wavelength dependent and those that are wavelength independent. The former are called chromatic aberrations and the latter are called monochromatic aberrations. In ordinary optical design, first the number and types of elements and their general configuration is chosen. Secondly, the powers, materials, thickness, and spacings of the elements are determined. These are usually chosen to control the chromatic aberrations and the Petzval curvature of the system, as well as the focal length, working distances, field of view, and aperture. In the third stage of the design process, the shapes of the elements are adjusted to correct the basic aberrations to desired values. In the fourth stage of the design process, the residual aberrations are reduced if necessary to an acceptable level. Often, in conventional optical design, a figure of merit such as contrast is used to optimize the design. In many cases, there are tradeoffs in the design. For example, in microscopes the objectives are optimized for a specific conjugate. As a consequence, when not operating at that conjugate imaging spherical aberration is introduced causing a degradation in both resolution and image quality. This in turn inhibits pattern recognition and tracking.
The invention described in this disclosure overcomes these obstacles by, for example, incorporating an optional tunable filter in conjunction with a spatial light modulator/stepper motor combination that acts as a programmable element that compensates for wavelength independent focus aberrations. When the tunable filter is used in conjunction with the spatial light modulator/stepper motor the depth of the field is increased for each wavelength. If the tunable filter is not used, broadband operation results in a significantly enhanced depth of field. This invention enables the depth of field to be significantly enhanced by compensating for the defocus aberrations associated with each photodetector element. The enhanced depth of field described in this specification allows all objects within the field of view to be imaged with up to diffraction limited quality. In conventional optics, one must reduce the F# to change the depth of focus. This invention does not require one to make this sacrifice.
Depth of focus rests on the assumption that for a given optical system, there exists a blur (due to defocusing) of small enough size such that it will not adversely affect the performance of the system. Depth of focus is the amount by which the image may be shifted longitudinally with respect to some reference plane (e.g. film, photodetector) which will introduce no more than the acceptable blur. The depth of field is the amount by which the object may be shifted before the acceptable blur is produced. The size of the acceptable blur may be specified as the linear diameter of the blur spot or as an angular blur. Many books discuss lens design with respect to depth of field. The invention disclosed here uses, for example, the spatial light modulator 40 to act as the adaptive (or active) optic element to compensate for the defocus.